(i) Field of the Invention
The present invention relates to wood-cased pencil leads and pencil leads for mechanical pencils.
More specifically, it relates to baked pencil leads having high strength as well as holding high density and smooth writing performance which are usually contrary to the improvement of the strength, and a method for preparing the baked pencil leads.
(ii) Description of the Prior Art
Conventional wood-cased pencil leads and pencil leads for mechanical pencils are manufactured by adding a binder such as clay, a natural polymer, a synthetic polymer, a pitch or an asphalt to a coloring agent such as graphite or carbon black, further adding a suitable solvent and plasticizer if necessary, kneading them, extruding the kneaded material into a linear shape, baking the extruded articles at a temperature of from 900.degree. to 1,200.degree. C., and then impregnating pores in the baked articles with an oil.
It is known that even in the conventional pencil leads, a filler such as flaky graphite is oriented in a tubular form in an outer peripheral portion of each lead owing to high shear stress caused by a die at the time of extrusion to heighten the strength of the lead. However, of the conventional baked pencil leads, particularly thin leads for mechanical pencils are poor in strength, and therefore their strength has not reached a practical level, depending upon the hardness of the leads.
In order to improve the strength of the leads, various techniques have been suggested, but usually when the strength of the leads is improved, their density lowers and writing performance also tends to deteriorate. Conversely, if the writing performance is improved, the strength is liable to decline. Hence, there has not been any fundamental solution of this problem.
FIG. 4 shows a cross-sectional matrix structure of the usual lead for the mechanical pencils particularly by exhibiting a distribution of graphite constituting flaky crystals. As shown in this drawing, in a skin layer (a surface layer portion) of the outer peripheral portion of the lead, a filler such as the flaky graphite is oriented in a concentric tubular form along the outer periphery, but in the internal matrix structure of the lead, the filler is oriented at random as a cluster of bees in a beehive.
Thus, much attention is paid to such a cross-sectional structure of the lead, and various techniques for improving this structure have been suggested.
In Japanese Patent Publication No. 33957/1987, it is disclosed that a material is extruded into a star-shaped profile linear form having a rough outer periphery through a first step nozzle, and this profile linear article is further extruded into a linear form having a predetermined diameter. This technique is still poor to orient the filler even in the internal portion of each lead.
Japanese Patent Publication No. 36880/1987 discloses that a material is extruded into a plurality of thin lines through a first step nozzle, and a group of these thin lines is then extruded into a linear form having a predetermined diameter.
This technique also intends to spread the orientation of the filler along the outer peripheral portion of the conventional pencil lead to the internal portion, but in fact, the initial orientation is disordered by the second step nozzle. Even if the initial orientation is maintained, the filler is oriented in a radial direction on half sides of the plural thin lines.
Japanese Patent Application Laid-open No. 139274/1992 discloses pencil leads in which a filler is oriented radially from the central portion to the outer peripheral portion of each lead in the cross-sectional view of the lead. This technique intends to increase the density of lines by a plotter or the like for drawing vertical lines, but since the filler is radially parallelly oriented on the whole, cracks easily occur along the parallel orientation of the filler by shrinkage stress extending from the outer peripheral portion to the central portion of each lead which is caused in a heat treatment step.